Wireless signal amplification system and a television signal decoder comprising one such system

ABSTRACT

A wireless signal amplification system including amplification apparatus ( 6 ) consisting of numerous different amplifiers ( 6   1  to  6   n ) which are distributed in an analogue processing chain ( 4 ). Also, a converter ( 12 ) for converting analogue signals into digital signals which, at input, are connected to the outlet of the analogue processing chain ( 4 ) and, at output, are connected to at least the gain control circuit ( 16 ) of the amplification apparatus ( 6 ) according to a value that is representative of a characteristic of the wireless signal ( 1 ). In this way, sampling of the wireless signal ( 1 ) by the converter ( 12 ) is optimized. The gain control circuit ( 16 ) establishes an average gain control signal ( 18 ), and also has, for each of the amplifiers ( 6   1  to  6   n ), a device for calculating an individual gain control signal ( 20   i  to  20   n ) according to a transfer function (H 1  to H n ) which is specific to each amplifier and which is applied to the average gain control signal.

The present invention relates to an amplification system for a wirelesssignal and to a television signals decoder comprising such a system.

The wireless transmission of signals presents certain technologicalconstraints.

The deterioration of such a signal on reception, the attenuation due tothe distance separating the receiver from the transmitter and thefiltering operations, lead to the obtaining of a utilizable signal oflow amplitude.

Therefore, processing chains always include amplification elements whosecalibration is fundamental to the utilization of the signal.

In particular, when the signal transmitted corresponds to a digitalcoding, such as, for example the OFDM (orthogonal frequency distributionmultiplexing) coding, it is frequency and amplitude modulated. Hence,the signal exhibits several amplitude levels representative of thedigital information transmitted that it is necessary to differentiate onreception in order to reconstruct the signal.

After a first processing of the signal in its analogue form, comprisingin particular steps of filtering and of amplification, the signal isconverted into a digital signal.

In order for the conversion to be as accurate as possible, the signalshould be amplified so that it occupies the entire input range of theconversion means.

Conventionally, the amplification is carried out by several amplifiersdistributed throughout the processing chain.

For example, a first amplifier operates at the signal carrier frequency,and another at the intermediate frequency at which the input signal ofthe demodulator is sampled.

Conventionally, the control of the gain of these amplifiers is carriedout with the aid of a loop for detecting the power of the input signalthat makes it possible to adjust the amplification.

However, this single control does not make it possible to control thegain of several independent amplifiers. This system can however beimproved by adding analogue components making it possible to distributethe total gain between the various amplifiers.

However, this does not offer perfect control, since the variousamplifiers are necessarily all in operation and the distribution of thegain remains the same throughout the signal amplification range.

Moreover, the conventional systems readily bring about the saturation ofone or other of the amplifiers of the chain, resulting in signaldistortion.

The present invention aims to remedy these problems by providing anamplification system equipped with control means making it possible foreach of the amplifiers of the system to be controlled individuallyaccording to their own control signal.

One subject of the invention is an amplification system for a wirelesssignal comprising amplification means consisting of a plurality ofdistinct amplifiers distributed within an analogue processing chain, andmeans for converting analogue signals into digital signals linked atinput, to the output of said analogue processing chain and at output, atleast, to means of control of the gain of said amplification means as afunction of a value representative of a characteristic of said wirelesssignal so as to optimize the sampling of said wireless signal by saidconverter, said control means comprising means for establishing a meangain 10 control signal, characterized in that said control meansfurthermore comprise, for each of said amplifiers, means for calculatingan individual gain control signal according to each amplifier's owntransfer function applied to said mean gain control signal.

According to other characteristics of the invention:

-   -   the means for establishing said mean gain control signal        comprise means of estimating the power of the wireless signal        received and means for determining a mean gain control signal        dependent on the power of said wireless signal;    -   said calculation means comprise for each amplifier, a table        defining the transfer function for going between the mean gain        control signal and the individual gain control signal        corresponding to said amplifier;    -   said transfer functions define for each amplifier, a succession        of linear gains between the mean gain control signal and the        individual gain control signals;    -   said transfer functions are determined successively such that        each of the amplifiers operates in a linear regime when the        others are locked at their minimum linear gain, so that the        overall gain of the amplification means is a linear gain;    -   at least one of said amplifiers forming said amplification means        has analogue control, the corresponding output of said control        means then being linked to the control input of this amplifier        though digital/analogue conversion means; and    -   at least one of said amplifiers forming said amplification means        has digital control, the corresponding output of said control        means then being linked directly to the control input of this        amplifier.

Another subject to the invention is a television signals decoder usingsuch an amplification system.

According to other characteristics of the invention:

-   -   the decoder is adapted for decoding the signals coded according        to the OFDM orthogonal frequency division multiplexing protocol.

The invention will be better understood on reading the description whichfollows, given merely by way of example and while referring to theappended drawings, in which:

FIG. 1 represents a schematic diagram of an amplification systemaccording to the invention;

FIGS. 2 and 3 diagrammatically represent the detail of the means ofcontrol of the amplifiers of an amplification system according to theinvention;

FIG. 4 represents an exemplary table defining the transfer function ofthe control signal for an amplifier in a system according to theinvention; and

FIG. 5 is a graphical representation of the transfer functions of eachof the control signals of the amplifiers of a system according to theinvention.

Represented in FIG. 1 is a system for processing a wireless signalequipped with an amplification system according to the invention.

A wireless signal 1 is received by an antenna 2.

For example, the signal 1 is a television signal transmitted accordingto the OFDM (orthogonal frequency division multiplexing) protocol.

The signal 1 is introduced into an analogue processing chain 4.

Conventionally, this processing chain 4 groups together severalfunctions and comprises a plurality of amplifiers 6 ₁ to 6 _(n), whichamplifiers constitute amplification means 6 in a conventional manner.

The amplifiers 6 ₁ to 6 _(n) may be included in functional elements ofthe processing chain 4 or else be interposed between these elements.

For example, the amplifier 6 ₁ is included in a tuner system and isseparated from the amplifier 6 ₂ by filtering means 8.

Furthermore, the processing chain 4 comprises, for example, conventionalmeans of frequency transposition 10.

It is therefore apparent that, in the configuration described, theamplifiers 6 ₁ and 6 ₂ operate at the carrier frequency of the wirelesssignal 1 and the amplifier 6 _(n) operates at an intermediate frequencyused for a sampling operation.

The output of the processing chain 4 is linked to analogue/digitalconversion means 12 that perform the sampling of the signal.

For example, these conversion means 12 are formed by a converterintegrated into an electronic component.

The digital signal obtained at the output of the analogue/digitalconverter 12 is transmitted to a demodulator 14 charged with theinterpretation thereof.

For example, the demodulator 14 is a demodulator of television signalscoded according to the OFDM protocol.

Control means 16 are also linked to the output of the converter 12 so asto be able to control the amplification means 6 as a function of theprocessed and digitized wireless signal 1.

The control means 16 comprise, at input, means for establishing a meangain control signal. This signal makes it possible to determine theoverall gain of the amplification means 6 so as to best utilize theinput range and the accuracy of the converter 12.

An example of detail of operation of these establishing means 18 isdescribed later with reference to FIG. 2.

Within the context of the invention, the mean control signal istransmitted to calculation means 20 ₁ to 20 _(n) which make it possibleto form, from the mean gain control signal, several individual controlsignals destined for each of the amplifiers 6 ₁ to 6 _(n).

These calculation means 20 ₁ to 20 _(n) are grouped together under thegeneral numerical reference 20.

For each amplifier 6 ₁ to 6 _(n), a transfer function H₁ to H_(n) isdefined for going between the mean gain control signal and theindividual gain control signals.

The obtaining and the implementation of one of these transfer functionsis described in greater detail with reference to FIG. 3.

Digital control signals specific to each of the amplifiers 6 ₁ to 6 _(n)are thus obtained.

These signals are then converted back into analogue signals bydigital/analogue conversion means 22 ₁ to 22 _(n) such as integratedconverters and transmitted to each of the amplifiers 6 ₁ to 6 _(n).

For example, the individual gain control signals are dc voltages varyingbetween 0 and 5 V, that control the gain of the amplifiers 6 ₁ to 6_(n).

In another configuration, the amplifiers 6 ₁ to 6 _(n) are adapted to becontrolled directly by digital signals thereby dispensing with the needto convert the individual digital control signals into individualanalogue control signals.

The system described therefore makes it possible to control each of theamplifiers 6 ₁ to 6 _(n) individually.

For example, if one wishes to avoid the saturation of the variousamplifiers, then amplifiers 6 ₁ to 6 _(n) are used, the range of whosesignal input level is greater and greater. Thus, the amplification means6 are controlled in such a way that a first amplifier is used for thevery weak signals, then the others are used in succession as and whenthe previous amplifiers reach the limit of their saturation zone and arelimited to this gain.

Described in FIG. 2 is the detail of the means for establishing the meangain control signal 18.

The establishing means 16 receive as input the digital signaloriginating from the converter 12.

This signal is squared so as to estimate its power by the powerestimator means 26.

This value is thereafter introduced into the comparator 28 whichsubtracts from it a fixed value α representing the theoretical meanpower.

This value α is fixed as a function of the parameters of the modulationused and of the converter 12; it corresponds to the optimal mean powerfor the sampling operation.

A filtering is thereafter performed by virtue of a conventional digitalaccumulation loop consisting of the multiplier 30 applying a coefficientβ in conventional manner, of the adder 32 and of the register 34, so asto determine a signal of mean deviation between the power of thewireless signal received 1 and a theoretical mean power.

This signal serves as gain control means signal and is transmitted tothe calculation means 20.

Represented in FIG. 3 are the elements constituting the calculationmeans 20 that make it possible to determine the transfer function of theamplifier 6 _(i).

These elements receive at input the mean gain control signal defined bythe establishing means 18.

This signal is injected into a comparator 36 _(i), as well as into means38 _(i) for determining the individual control signal of the amplifier 6_(i).

The comparator 36 _(i) compares the mean gain control signal with anoperating table 40 _(i) specific to the amplifier 6 _(i).

An exemplary operating table is given with reference to FIG. 4.

This table 40 _(i) makes it possible to determine the transfer functiondefining the individual signal for control of the gain of the amplifier6 _(i) as a function of the value of the mean gain control signal.

The value of the mean gain control signal has been arbitrarilynormalized between 0 and 1 and this range has been arbitrarily dividedinto five equal intervals, over which the coefficients 0,1,3,1 and 0respectively are applied.

Thus, if the mean gain control signal has a normalized value of 0.5, thecomparator 36 _(i), with the aid of the table 40 _(i), determines thetransfer function to be applied and transmits this value to thedetermination means 38 _(i).

The latter then apply, in this example, an affine function with directorcoefficient 3 to the mean gain control signal so as to obtain theindividual gain control signal for the amplifier 6 _(i).

An individual gain control signal intended for the amplifier 6 _(i) isthus obtained from the mean gain control signal and from the operatingtable 40 _(i) specific to the amplifier 6 _(i).

This signal is thereafter transmitted to the digital/analogue converter22 _(i) and then to the amplifier 6 _(i).

Of course, a set of similar elements is disposed on each circuit forcontrol of the amplifiers 6 ₁ to 6 _(n).

The calculation means 20 therefore comprise comparators 36 ₁ to 36 _(n),means of determination 38 ₁ to 38 _(n) and operating tables 40 ₁ to 40_(n).

However, it is conceivable to control several amplifiers in an identicalmanner by using one and the same group of control elements.

Represented graphically in FIG. 5 is the curve H_(i) representative ofthe transfer function stemming from the table 40 _(i) described withreference to FIG. 4.

By way of example, also represented are the curves H₁ and H_(n)corresponding to the transfer functions describing the individualsignals for gain control of the amplifiers 6 ₁ and 6 _(n), as a functionof the mean gain control signal.

The operating tables 40 ₁ to 40 _(n) are defined initially as a functionof the overall gain to be obtained for the amplification means 6.

Specifically, the overall gain of the amplification means 6 is equal tothe sum of the gains of each of the amplifiers 6 ₁ to 6 _(n). Thesegains are functions of the individual gain control signals themselvesresulting from the application of the transfer functions H₁ to H_(n) tothe mean gain control signal.

For example, to determine the transfer functions H₁ to H_(n) in the casewhere one wishes to obtain a linear overall gain, all the amplifiers 6 ₁to 6 _(n) are configured to the minimum gain of their linear zone,except one, then the input signal range 1 is determined, as hence is themean gain control signal range enabling this amplifier to operatelinearly.

Outside of this zone, the individual gain control signal is such thatthe gain of this amplifier is constant.

These operations are repeated successively for all the amplifiers so asto obtain the set of transfer functions H₁ to H_(n).

The transfer functions H₁ to H_(n) have thus been defined successivelyso that each of the amplifiers 6 ₁ to 6 _(n) operates in a linear regimewhen the others are locked at their minimum linear gain, so that theoverall gain of the amplification means 6 is a linear gain.

It is therefore apparent that the present invention makes it possible tomaximize the amplification control range for a given set of amplifiers.

In particular, it is possible to amplify signals of very low amplitudewithout distortion.

Specifically, the present invention makes it possible not to reach thesaturation zones of the various amplifiers of the chain, while retainingcontrol of the overall gain of the amplification means.

In particular, one can thus manage to define a linear overall gain whileavoiding saturations of the signal at the converter level.

Such control means 16 are, in conventional manner, readily implantable,in digital fashion, in an integrated circuit.

Although one embodiment has been described, it is not regarded aslimiting the scope of the present invention.

Specifically, the invention can also make it possible to define anoverall gain that is not linear so as, for example, to correct a defecton transmission.

In the same way, such a system may be configured in such as way as tocompensate for the defects of certain amplifiers.

Moreover, the example chosen implemented a television signal codedaccording to the OFDM protocol; of course, the present invention isapplicable to types of wireless signals other than those mentioned.

Likewise, within the context of the use of the present invention in anOFDM demodulator, it is conceivable to use the channel estimator in thecontrol means so as to determine the power of the wireless signalreceived.

It is also conceivable to use means of adapting the coefficients α and βas a function of the type of modulation implemented and of the type ofconverter used.

1. An amplification system for a wireless signal (1) comprisingamplification means (6) consisting of a plurality of distinct amplifiers(6 ₁ to 6 _(n)) distributed within an analogue processing chain (4), andmeans (12) for converting analogue signals into digital signals linkedat input, to the output of said analogue processing chain (4) and atoutput, at least, to means (16) of control of the gain of saidamplification means (6) as a function of a value representative of acharacteristic of said wireless signal (1) so as to optimize thesampling of said wireless signal (1) by said converting means (12), saidcontrol means (16) comprising means for establishing a mean gain controlsignal (18), characterized in that said control means (16) furthermorecomprise, for each of said amplifiers (6 ₁ to 6 _(n)), means forcalculating an individual gain control signal (20 ₁ to 20 _(n))according to each amplifier's own transfer function (H₁ to H_(n))applied to said mean gain control signal.
 2. The amplification system asclaimed in claim 1, characterized in that the means for establishingsaid mean gain control signal (18) comprise means (26) of estimating thepower of the wireless signal received (1) and means for determining amean gain control signal dependent on the power of said wireless signal(1).
 3. The amplification system as claimed in claim 1, characterized inthat said calculation means (20 ₁ to 20 _(n)) comprise for eachamplifier (6 ₁ to 6 _(n)), a table (40 ₁ to 40 _(n)) defining thetransfer function (H₁ to H_(n)) for going between the mean gain controlsignal and the individual gain control signal corresponding to saidamplifier.
 4. The amplification system as claimed in claim 3,characterized in that said transfer functions (H₁ to H_(n)) define foreach amplifier (6 ₁ to 6 _(n)), a succession of linear gains between themean gain control signal and the individual gain control signals.
 5. Theamplification system as claimed in claim 1, characterized in that saidtransfer functions (H₁ to H_(n)) are determined successively such thateach of the amplifiers (6 ₁ to 6 _(n)) operates in a linear regime whenthe others are locked at their minimum linear gain, so that the overallgain of the amplification means (6) is a linear gain.
 6. Theamplification system as claimed in claim 1, characterized in that atleast one of said amplifiers (6 ₁ to 6 _(n)) forming said amplificationmeans (6) has analogue control, the corresponding output of said controlmeans (16) then being linked to the control input of this amplifierthough digital/analogue conversion means (22 ₁ to 22 _(n)).
 7. Theamplification system as claimed in claim 1, characterized in that atleast one of said amplifiers (6 ₁ to 6 _(n)) forming said amplificationmeans (6) has digital control, the corresponding output of said controlmeans (16) then being linked directly to the control input of thisamplifier.
 8. A television signals decoder, characterized in that itcomprises an amplification system as claimed in claim
 1. 9. The decoderas claimed in claim 8, characterized in that it is adapted for decodingthe signals coded according to the OFDM orthogonal frequency divisionmultiplexing protocol.